The present invention relates generally to earth boring bits of the type used to cut a shaft in the earth, and specifically to large diameter earth boring bits of the type used in the mining and construction industries with variable effective cutting diameters to cut shafts of various sizes.
Large diameter earth boring bits are presently used in the mining and construction industries which vary in overall diameter from between about 2-3 feet to 20 feet or more in diameter. Typical prior art configurations for such bits included in a drill string, an intermediate mandrel connected at one end in the drill string and having an opposite end adapted to be connected to a drill bit body. In certain configurations, the mandrel opposite end was provided with a circular flange which matingly engaged the upper surface of the drill bit body. The bit body was provided with a plurality of roller cutting elements affixed by means of permanent mountings to the bottom surface of the bit body. A series of weights were often stacked above the mandrel flange to provide a load on the downwardly facing roller cutters carried on the bit body and one or more stabilizer structures were also often mounted above the bit body around the mandrel and/or about the bit body itself.
The driving torque for the cutting elements in such prior configurations was typically transmitted through the drill pipe, through the mandrel, and through the mandrel flange which was typically bolted to the upper surface of the bit body to cause rotation of the bit body and, in turn, rotation of the roller cutting elements.
While such arrangements were effective for cutting large diameter shafts many feet in diameter, the large diameter bits were extremely costly. Because the cutting elements were affixed by means of permanent mountings to the bit body, it was necessary to provide a separate bit for each different sized diameter hole or shaft which was to be cut.
There has existed a need, therefore, for a large diameter earth boring bit which could conveniently be expanded or contracted in size whereby the effective cutting diameter of the cutting elements could conveniently be increased or decreased without replacing the entire bit body.